Roofs having slopes approaching the horizontal have special problems in the prevention of leakage. This roof configuration is common in commercial buildings, and thus generates considerable maintenance. Typically, such a roof will slope at one foot or less per 12 feet of horizontal distance. Rain water or thawing ice is the source of the leakage, and this is compounded by the presence of remaining snow and ice that can interfere with expected drainage. So-called "membrane roof construction" was intended to correct the leakage problem, but produced its own set of problems with the passage of time. A membrane roof normally includes some form of truss system for support, and a layered sub-roof assembly extending between the support points. A film of water-impervious material (either in sheet form, or poured from initially-liquid material) is placed on top of the sub-roof. All is fine until a leak occurs somewhere. The film is supposedly protected by "ballast" material, which commonly is in the form of crushed stone or gravel. A service man walking across the roof can easily and inadvertently punch one of the stones through the film. The expected shrinkage and expansion of the roof components can also induce small ruptures in the film.
Leakage through one of these film discontinuities seems to intentionally defy attempts to locate it for repair. After moving laterally along the underside of the roof components, it can easily become first visible in the walls of the building, perhaps 30 feet from the location of the leak. Where the lateral flow takes place between the layered roof components, it may be necessary to tear off a large section of the roof to find it. This is particularly a problem when corrugated sheet material is used for bridging across between the support members. Insulation panels usually are laid over the corrugated sheet, forming concealed channels for the movement of water. Even noncorrugated layers of a roof have a tendency to provide minute passages between them for the concealed lateral movement of leakage before it becomes detectable. It must be kept in mind that the surface tension of wate will enable it to cling to the underside of a roof component, as well as ride along on top of it.